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a refresher, start
Definitions: cold runner: a feed system for the flowing of plastic into individual cavities, which is ejected with the parts after it, solidifies. Terminology: the terms used in a specified field of study, or industry.
While many assume most know all the terminology of the runner it has become apparent that at times a refresher is necessary.
Figure 1
The above illustrations shows design for an eight cavity mold which is naturally balanced. Though this is a simple design and is common some minor explanations may help comprehension as to what is going on.
Sprue:
This as shown is composed of a taper area and at the tip is the O dimension which is to mate with the nozzle of the machine, and is the first point of plastic flowing into the feed system. The sprue has a taper, which can be specified per material for its construction of taper and minimum O dimensions. Also it should be understood that the longer the sprue is the greater in diameter it shall be at its mating position to the Primary runner.
In many cases the diameter at the junction between the sprue and the primary runner is too great and is the one point of controlling the cycle time as the cycle is waiting for this to be cool enough( solid enough) so as to be able to be ejected. There is also a choice of sprue pullers that may be incorporated into the design so as to assure that the sprue is pulled out. This puller area below the primary runner but in the same direction as the sprue also incorporates a cold slug well in many cases.
Primary Runner:
This is the first flow of the material past the sprue, and in the illustration above it divides the flow in half, meaning the flow is going to the right and left as shown in the illustration. The primary runner is the 1st runner material flows through.
Secondary Runner:
This is the second flow path taken after having passed through the 1st, there is typically a reduction in approximate diameter of the flow path. If as has been shown it splits in two than again ½ goes in one direction and ½ goes in the other direction.
Tertiary runner:
This is the 3rd runner the material flow s through and again as in the secondary runner its approximate diameter of the flow path is reduced.
Please note there may in some cases be more branches of runners which become the 4th 5th etc. All of which see a reduction in cross sectional area. The key here is that even as they decrease in size they are in relation to the part wall thickness of a greater size so as to supply ample material to the part and to not freeze off prior to packing of the part. Typically in design the final runner diameter the first diameter to be sized and the runners before it in the flow system are increased in size to provide for the flow and splitting of flow to the part.
The final area in the runner system is the gate area, which is sized per part wall thickness and the type of material to be processed. Further the overall dimensions of the gate must be related also to the fill rate of plastic material being used. This shall be covered in detail later.
Cold Slug well:
Commonly referred to an area under the sprue it is often incorporated at turns in the runner. The cold slug is the first material to enter the sprue, and is thought to be too cold to process. In an abstract way it is like a bullet in a gun and there is only one, yet experience has shown the cold slug shall go where ever it wants and not necessarily where one place such cold slug wells.
In the above illustration it is referred to as naturally balance. What this means is that the flow distance to each gate area/part is the same. It assumes that the runner diameters are the same at the various segments.
Understanding principles: naturally balanced runner.
In understanding the plastic flow is the same to each gate, one than would also expect that the flow of material is first divided at the primary, than again at the secondary and then again at the tertiary runner. Thus if one were to block off one gate in the above example it would change the fill rate of its opposite part, which may or may not cause issues with quality and performance.
In any runner system, which basically feeds the materials to the part, blocking a gate, changes the flow and pattern of filling to the remaining cavities and should always be a consideration and concern prior to doing so.
SlSilvey
11082014.01
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